dh_systemd_start is a debhelper program that is responsible for
starting/stopping or restarting systemd unit files in case no corresponding
sysv init script is available.

As with dh_installinit, the unit file is stopped before
upgrades and started afterwards (unless --restart-after-upgrade is
specified, in which case it will only be restarted after the upgrade). This
logic is not used when there is a corresponding SysV init script because
invoke-rc.d performs the stop/start/restart in that case.

Do not stop the unit file until after the package upgrade has been
completed. This is the default behaviour in compat 10.

In earlier compat levels the default was to stop the unit file
in the prerm, and start it again in the postinst.

This can be useful for daemons that should not have a possibly
long downtime during upgrade. But you should make sure that the daemon
will not get confused by the package being upgraded while it's running
before using this option.

--no-restart-after-upgrade

Undo a previous --restart-after-upgrade (or the default of compat
10). If no other options are given, this will cause the service to be
stopped in the prerm script and started again in the
postinst script.

-r, --no-stop-on-upgrade,
--no-restart-on-upgrade

Do not stop service on upgrade.

--no-start

Do not start the unit file after upgrades and after initial installation
(the latter is only relevant for services without a corresponding init
script).

Note that this command is not idempotent. dh_prep(1) should be called
between invocations of this command (with the same arguments). Otherwise, it
may cause multiple instances of the same text to be added to maintainer
scripts.

Note that dh_systemd_start should be run after
dh_installinit so that it can detect corresponding SysV init scripts.
The default sequence in dh does the right thing, this note is only
relevant when you are calling dh_systemd_start manually.